1. Field Of The Invention
The present invention relates to a multiple value image input device which receives picture images in multiple chromatic grades or tonal ranges. The invention particularly relates to a multiple value image input device able to render the varying shades of picture images in clear definition.
2. Discussion Of The Related Art
Picture image processing with a computer system is now used extensively. Most presently known picture image input devices convert the input picture images into binary signals. The input data can be received from a scanner, a facsimile machine, a computer or some other device. A typical known image input device is the picture image input block in the conventional facsimile machine. However, the block image data processing method cannot reproduce halftone images even if it can reproduce characters and line drawings. Halftone recording processes such as the "Dither Process" have been developed, and multiple value picture image input devices, which are capable of performing the input of picture image data in multiple values, are used extensively.
In the known picture image input device, shading correction is applied to the picture image data obtained with the image input device. Specifically, the levels of the signals output by the pixels are not in complete agreement. Even if an identical part of an original sheet with the same degree of optical density is read, various factors such as non-uniformity in the sensitivity of the individual pixels comprising the one-dimensional image sensor and non-uniformity in the quantity of light on the lines of the original sheet will create inconsistent readings.
To correct inconsistent readings, a known image input device is provided with a plate with a white color (the density indicating block) arranged in the main scanning direction of a one-dimensional image sensor outside the original sheet reading area. The one-dimensional image sensor reads the white plate before reading an original sheet. The signal level is then adjusted so that the value read by each of the individual pixels will be in agreement for the level of the white color.
Thus, the conventional multiple value image input device performs density correction in picture images with reference to a white plate arranged in the main scanning direction of the one-dimensional image sensor. However, this process cannot provide a satisfactory range of chromatic grades. If an original sheet showing a human face portrayed on a large scale is read by a multiple value image input device which can reproduce many chromatic grades, such as 64 chromatic grades, the conventional process will reproduce the area of the facial skin in monotonous expression. The conventional process will only use a narrow range of the available chromatic grades to express the face. It is then difficult to recognize the facial expression and the characteristic features of a face reproduced by the known process as compared with direct observation of the original sheet. The same problem is also found with a photograph taken in twilight without a flash lamp, a landscape veiled in a thick morning mist, or an old original sheet with faded color.